The invention relates to a display apparatus comprising a heater circuit. The invention also relates to a method of generating a heater voltage.
In a display apparatus with a cathode ray tube, a heater voltage is generated to heat a heater of the cathode ray tube. U.S. Pat. No. 4,481,476 discloses a circuit for generating a heater voltage which has a nominal value during a normal operating phase, and which has a boost value which is higher than the nominal value during a boost period of a start-up phase. In this way it is possible to shorten the time required to heat the heater to a temperature which enables display of information on the cathode ray tube with adequate quality. The circuit comprises a transformer and a resistor with a positive temperature coefficient (further referred to as PTC). During the start-up phase, the PTC is cold and has a low impedance, two windings of the transformer are connected in series and a high heater voltage is supplied. During the normal operation phase, the PTC is hot and only one winding supplies the heater voltage which has the nominal value now. To reach a high enough impedance value in the normal operation phase, the PTC becomes extremely hot, and consequently, other components have to be positioned at a save distance from the PTC.
It is an object of the invention to provide an improved heater circuit enabling a quick start of a picture display on a cathode ray tube, while the components of the heater circuit become less hot.
To this end, a first aspect of the invention provides a display apparatus comprising a heater circuit that includes a stabilizing power supply (SPS) for supplying the heater voltage (Vh) having a value depending on a control signal (Cs). The heater circuit also includes a timing circuit (TC) for supplying the control signal (Cs) determining the boost period (Tb). A second aspect of the invention provides a heater circuit that includes a stabilizing power supply (SPS) for supplying the heater voltage (Vh) having a value depending on a control signal (Cs). The heater circuit also includes a timing circuit (TC) for supplying the~.control signal (Cs) determining the boost period (Tb). A third aspect of the invention provides a method of generating a heater voltage by a step of stabilizing (SPS) the heater voltage (Vh) for supplying the heater voltage (Vh) having a value depending on a control signal (Cs), and a timing step (TC) for supplying the control signal (Cs) determining the boost period (Tb). Some advantageous embodiments of the invention are defined in the dependent claims and in the disclosure that follows.
The heater circuit according to the invention comprises a stabilizing power supply with a control input which receives a control signal to control the value at which the heater voltage has to be stabilized. For example, in stabilizing power supplies which are controlled by an error amplifier which compares on the one hand a feedback signal depending on the heater voltage with on the other hand a reference signal, the control signal may influence the feedback signal or the reference signal. As is known from series regulator power supplies, the error amplifier may control the voltage level at a control electrode of a semiconductor element to stabilize the output voltage of the stabilizing power supply. As is known from switched mode power supplies, the error amplifier may control an on-time of a semiconductor element.
A control circuit generates the control signal such that in the normal operation phase the stabilizing power supply generates the heater voltage with a nominal value, and such that during a boost period of a start-up phase, the stabilizing power supply generates the heater voltage with a boost value higher than the nominal value. The normal operation phase is defined as the phase during which the nominal heater voltage has a value such that the temperature of the heater is sufficiently high to enable display of a picture with good quality, and such that the heater has a long lifetime. The start-up phase is the phase during which the display apparatus starts up and the heater warms up.
The heater circuit according to the invention does not contain any components which have to reach a certain high temperature to reach a nominal value of a heater voltage after the boost period. The temperature of components which might dissipate considerable power may be kept low by applying heatsinks. It is impossible to apply a heatsink to the PTC, because the PTC has to reach the high temperature to obtain the high impedance.
It is known from multi-frequency display apparatus, which have to display signals with different line scan frequencies, to generate a stabilized heater voltage which is independent of the line scan frequency. It is a drawback of this known heater voltage generation that it takes a considerable time before the heater is hot enough to display a good-quality picture.
The boost period is automatically generated in response to a switch-on instant which determines a start of the start-up phase. For example, the start-up phase may be initiated by switching-on a display apparatus by pressing a mains switch, or by making the display apparatus start the normal operation phase when in a standby mode.
The boost period may also start a certain period of time after the start of the start-up period to avoid damage to the heater due to hot spots.
The shape of the heater voltage is controlled to obtain a constant power dissipation in the heater during the start-up phase. In this way the heater is heated up as fast as possible without causing damage due to hot spots. The power dissipation in the heater during the start-up phase, and thus also during the boost period, is selected to be larger than the power dissipation in the heater during the normal operation phase succeeding the start-up phase.
The current in the heater is limited during the start-up phase to prevent damage to the heater due to hot spots.
These and other aspects of the invention will be apparent from and elucidated with reference to the accompanying drawings.